In recent years, development of displays has been remarkable. A liquid crystal display (LCD), an electroluminescence display (EL), a field emission display (FED), or the like is used as a display device for business machines such as a personal computer and a word processor, and a display device for control systems in factories. These displays have a sandwich structure in which a display device is held between transparent conductive oxides.
A main stream material for such a transparent conductive oxide is indium tin oxide (hereinafter may abbreviated as “ITO”) prepared by a sputtering method, an ion plating method, or a vapor deposition method as described in Non-patent Document 1.
ITO is composed of a specific amount of indium oxide and tin oxide, possesses excellent transparency and conductivity, can be etched using a strong acid, and exhibits high adhesion to a substrate.
Although ITO has excellent properties as a material for transparent conductive oxide, ITO is not only a scarce resource, but also contains a large amount (about 90 atomic percent) of indium which is a biologically harmful element. Moreover, the indium itself produces nodules (projections) during sputtering. The nodules produced on the target surface have been one of the causes of abnormal electrical discharge. In particular, when an amorphous ITO film is produced for improving etching properties, the indium compound on the surface of the target is reduced due to introduction of a small amount of water and hydrogen gas in the sputtering chamber, giving rise to further production of nodules. If an abnormal electrical discharge occurs, scattered materials become attached to the transparent conductive oxide as impurities during or immediately after the film formation.
The indium content in ITO must be reduced due to these problems of instability of supply (scarcity) and hazardous properties. However, the maximum solid solubility limit of tin oxide to indium oxide is considered to be about 10%. If the content of indium in ITO is reduced to 90 atomic percent or less, tin oxide remains in the target in the form of a cluster. Since the resistance of tin oxide is 100 or more times stronger than the resistance of ITO, the remaining tin oxide causes charges to accumulate during sputtering, which causes arcing, destroys the target surface, causes small fragments to scatter, and generates nodules and particles (Non-patent Document 2). Therefore, it is difficult to reduce the content of indium to 90 atomic percent or less.
As a method for preventing generation of nodules and suppressing abnormal electrical discharge, a hexagonal layered compound of In2O3(ZnO)m, wherein m is an integer of 2 to 20, with a crystal grain diameter of 5 μm or less has been investigated (Patent Documents 1 and 2). However, if the indium content is reduced to 90 to 70 atomic percent or less in this method, there are problems such as decrease of the sintered density and conductivity of the target, which causes abnormal electrical discharge and retards the film forming speed; low target strength, leading to easy cracking; and poor heat resistance in the presence of air of the transparent conductive film formed by sputtering. Moreover, a high temperature is required in order to produce the hexagonal layered compound in a stable manner. This brings about another problem of a high industrial manufacturing cost. Furthermore, the hexagonal layered compound has no resistance to an etching solution containing phosphoric acid used for etching of a metal or an alloy. Thus etching of the metal or alloy membrane formed on the film of the hexagonal layered compound is difficult.
As a transparent conductive film with a significantly reduced indium content, a transparent conductive film containing zinc oxide and tin oxide as major components has been studied (Patent Document 3). However, this method has problems such as difficulty in sputtering due to unduly high target resistance and a tendency of easy generation of abnormal electrical discharge. No study of a sputtering target to solve these problems has been undertaken.
Patent Document 4 discloses a zinc oxide sintered body doped with different kinds of elements containing a spinel structure (ZnX2O4, wherein X is an element with a positive trivalent or higher valency). However, no studies have been done on the effect of a sintered body having both a spinel structure compound of Zn2SnO4 and a bixbyite structure compound of In2O3.
Although the ITO target which is a sintered body essentially consisting of a bixbyite structure compound of In2O3 has been studied (Patent Document 5), Sn3In4O12 and the like are easily produced in the ITO target. Even if a sintered body essentially consisting of a bixbyite structure compound could be produced, only a narrow range of manufacturing conditions is allowed. Stable production of such an ITO target has been difficult. If the content of indium is reduced, production of a sintered body essentially consisting of a bixbyite structure compound is difficult.
It is known that in the IZO target which composed of indium and zinc, a hexagonal layered compound of In2O3(ZnO)m, wherein m is an integer of 2 to 20, is formed when the Zn content is 15 to 20 atomic percent (Non-patent Document 3). These crystal types have an effect of decreasing the target resistance and increasing the relative density as compared with ZnO. However, if the amount of indium is reduced (or the amount of zinc is increased), problems such as a decrease in the target strength and retardation of the film-forming speed may occur by increased target resistance and described relative density.
On the other hand, due to the progress of liquid crystal display functions in recent years, it has become necessary to use an electrode substrate with a metal or an alloy disposed on a transparent conductive film, such as an electrode substrate for a semi-transmissive, semi-reflective liquid crystal.
Patent Document 6 discloses a liquid crystal display device which has a transmissive region and a reflective region on a transparent conductive film. Patent Document 7 discloses that a film forming-etching process can be simplified by using a transparent conductive film possessing selective etching properties, that is, a transparent conductive film which is etched by an acid not corrosive to a metal, but is resistant to and etched only with difficulty by an etching solution used for etching metals. However, the method of Patent Document 7 may change the crystallization temperature and the work function of a transparent conductive film due to the addition of lanthanoids. Moreover, it is necessary to add lanthanoid oxide, which is a scarce resource, to the transparent conductive film in order to adjust the etching rate. Furthermore, there is almost no reduction in the indium content of the transparent conductive film.    [Patent Document 1] WO 01/038599    [Patent Document 2] JP-A-06-234565    [Patent Document 3] JP-A-08-171824    [Patent Document 4] JP-A-03-50148    [Patent Document 5] JP-A-2002-030429    [Patent Document 6] JP-A-2001-272674    [Patent Document 7] JP-A-2004-240091    [Non-patent Document 1] “Technology of Transparent Conductive Film” edited by The 166th Committee of Transparent Oxide and Photoelectron Material, Japan Society for Promotion of Science, Ohmsha, Ltd. (1999)    [Non-patent Document 2] Ceramics, 37 (2002), No. 9, pp 675-678    [Non-patent Document 3] Journal of the American Ceramic Society, 81(5), 1310-16 (1998))
An object of the invention is to provide a target with low resistance, high theoretical relative density, and high strength, a target with a reduced indium content, a target which allows stable sputtering while suppressing abnormal electrical discharge generated when forming a transparent conductive film by the sputtering method, a method for producing the targets, and a transparent conductive film produced using the sputtering targets.
Another object of the invention is to provide a transparent conductive film which can be selectively etched relative to a metal or an alloy, that is, a transparent conductive film which is etched by an acid not corrosive to a metal or an alloy, but is resistant to or is hardly etched by an etching solution used for etching the metal or alloy.
Still another object of the invention is to provide a transparent conductive film exhibiting a small increase in resistance during a heat treatment in the atmosphere and a small resistance distribution in a large area.
A further object of the invention is to provide a transparent electrode and an electrode substrate made from these transparent conductive films.
A still further object of the invention is to provide a simplified method for producing an electrode substrate from these transparent conductive films.